In a previous study, we have shown that the cryopreservation of mouse oocytes caused increases in the rates of degeneration and of digynic polyploid embryos, while the fertility of frozen-thawed oocytes was decreased. In this study, we have attempted to determine the different stages in the complete freezing-thawing process which are deleterious for the oocytes and the subsequent zygotes. IVF assays showed that DMSO decreased the fertility of oocytes, whereas cooling to 0 degrees C had no effect. DMSO, used at 0 degrees C, was less deleterious for oocytes. Thus, the prefreezing manipulations seem to be important for the quality and fertility of oocytes. However, neither DMSO nor cooling increased the incidence of chromosomal abnormalities in embryos obtained from inseminated exposed oocytes. Therefore, the increased frequency of polyploidy observed in embryos after the cryopreservation of mouse oocytes must correspond to disruption occurring during the freezing-thawing process.